A carburetor is provided in internal combustion engines for driving work apparatus such as chain saws, blower/vacuum apparatus, brushcutters or the like. The carburetor has an intake channel via which combustion air is inducted. In the carburetor, an air/fuel mixture is prepared and is supplied to the crankcase of an internal combustion engine. The intake operation is discontinuous because the intake stroke of an engine lies only approximately in the region smaller than a half crankshaft rotation. During this time, the piston of a two-stroke engine clears an inlet window via which the air/fuel mixture flows into the crankcase. After the end of the intake operation, the piston closes the inlet window so that the intake operation is interrupted for a time. Comparable conditions arise in a four-stroke engine because of the opening and closing of the connection between the cylinder and the intake channel by means of a valve.
A pressure wave develops reflecting at the closed inlet window because of the inertial and elasticity forces of the column of the air/fuel mixture in the intake channel which is accelerated in the intake direction. The pressure wave runs through the intake channel opposite to the intake direction. This pressure wave can run up to the intake opening at the opposite end of the intake channel especially for a fully open position of the throttle flap in the carburetor and propagate, for example, in the interior space of an air filter housing connected ahead. Fuel droplets of the air/fuel mixture, which is formed in the carburetor, can be entrained by this pressure wave opposite to the intake direction and be transported out of the intake channel. The fuel portion, which reaches the interior space of the air filter, leads there to coking which restricts the capacity of the air filter. The intake opening of the intake channel forms an open, likewise reflection capable end for the vapor column in the intake channel. The vapor column can oscillate in the intake channel and, at a specific rpm, resonance can occur which amplifies the unwanted effect of the fuel discharge.
To avoid the intake of fuel into the air filter housing, carburetor arrangements are known wherein a baffle wall is mounted at a spacing ahead of the intake opening. The baffle wall covers at least a portion of the intake opening. The pressure wave, which exits from the intake opening, is partially reflected by the baffle wall and is thrown back into the intake channel together with the entrained fuel droplets. These droplets cannot be braked without deceleration because of the relatively high specific weight of the fuel droplets compared to air whereby a portion of the fuel droplets impinges upon the baffle wall and there runs down. However, it has been shown that the impinging fuel droplets tend to burst upon impact and the resulting smaller fuel droplets are entrained by a part of the pressure wave past the baffle wall and into the interior space of the air filter housing.
It is an object of the invention to improve a known carburetor arrangement so that the entry of fuel into the air filter housing is minimized.
The carburetor arrangement of the invention is for an internal combustion engine driving a work apparatus. The carburetor arrangement includes: a carburetor; an intake channel defining a longitudinal axis and being formed in the carburetor and having a first end connected to the engine; the intake channel having a second end facing away from the engine and the intake channel being an intake opening at the second end; a baffle wall at least partially covering the intake opening; the baffle wall being mounted transversely to the longitudinal axis and being disposed at a distance from the intake opening; the baffle wall delimiting a baffle enclosure disposed between the baffle wall and the intake opening; and, the baffle wall having at least a first component part so inclined at an angle (xcex31) referred to the longitudinal axis that fuel droplets, which exit from the intake channel and impinge upon the first component part, form a film of fuel.
For this purpose, at least a first component piece of the baffle wall or the baffle wall as a whole is inclined at an angle of inclination with respect to the longitudinal axis of the intake channel. The angle of inclination is so selected that the fuel droplets, which exit from the intake channel and are entrained by the reflected pressure wave, do not burst upon impact but deposit as a fuel film on the baffle wall. This film of fuel can be conducted away in a defined manner from the baffle wall and again be introduced into the intake channel. By avoiding a bursting of the fuel droplets upon impact, the reflection of individual fuel droplets, which arise at bursting, from the baffle wall is avoided and therefore also the entry thereof into the interior space of the air filter. In this way, a coking of the air filter is avoided and, with the return of the fuel film from the baffle wall into the intake channel, a loss of fuel is likewise avoided.
In an advantageous embodiment of the invention, an end wall is provided which connects the lower edge of the baffle wall to a further lower edge of the intake opening. The end wall is, relative to the direction of gravity of the work apparatus (held in the usual operating position) disposed below the baffle wall. As a consequence of the above, the film of fuel can flow off from the baffle wall to the end wall supported by gravity. At the end wall, the film of fuel is transported back into the intake channel without supporting measures, such as a pump or the like, by the intake air flow which flows by. Suitably, the above-mentioned first component part of the baffle wall is inclined with respect to the longitudinal axis in such a manner that the cross section of the baffle space expands from the longitudinal axis in the direction of the end wall. Because of this kind of an alignment of the inclination angle, the kinetic energy of the fuel droplets, which exit from the intake opening, operates upon impact on the baffle wall with a component in the direction of the end wall. This force component acts supportively on the fuel film in the direction of gravity and, as a consequence thereof, an improvement of the conveyance of the fuel film in the direction of the end wall and from there into the intake channel is given without additional assisting means. The transition between the end wall and the baffle wall is configured so as to be rounded, whereby a defined air flow can form in the round, which, in this region too, makes possible a clean conveyance of the fuel film to avoid unwanted collection locations. The end wall extends advantageously uniformly and especially as a straight line into the lower channel wall of the intake channel whereby the fuel film can be carried into the intake channel with little resistance. Furthermore, the flow resistance of the inducted air is reduced in this manner.
In an advantageous further embodiment of the invention, a further wall is provided on each side of the baffle wall and the end wall and these walls conjointly define a baffle receptacle having an inflow opening lying opposite to the end wall. With a baffle receptacle of this kind, precisely defined flow conditions are given with simple means for the intake operation as well as for the reflection operation. As a consequence thereof, the induction resistance can be adjusted to be low and the inclination of the baffle wall for film formation can be adjusted precisely in a defined manner. The above-mentioned side walls define a rounded transition to the baffle wall and also to the end wall because a portion of the fuel film also can deposit on these walls. This region of the fuel film can be removed in the rounds correspondingly well by the intake air flow. In total, a further reduction of the intake flow resistance can be achieved with the rounded configuration.
In an advantageous further embodiment, the baffle wall includes a second component part which is inclined with respect to the longitudinal axis comparable to the above-described first component part for forming a fuel film. The angle of inclination of the second component part is so selected that the cross section of the baffle receptacle expands in the direction of the inflow opening starting from the longitudinal axis. In this way, the situation is accounted for that, in the region of the inflow opening, the pressure wave, which exits from the intake channel, together with the entrained fuel droplets, has a velocity component at an angle to the longitudinal axis of the intake channel in the direction of the inflow opening. The second component part is inclined oppositely to the first component part. The second component part leads, in this way, to a low angle of incidence of the fuel droplets also in this region whereby a formation of a film of fuel is facilitated also near the inflow opening. The cross section of the baffle receptacle expands in the direction of the inflow opening and furthermore leads to a reduced intake resistance. In a further embodiment, the above-mentioned component parts border on one another and form an edge. In this way, a transition region is avoided wherein fuel droplets can impinge at an obtuse angle. A formation of a fuel film in the entire region of the baffle wall is thereby facilitated.
Analytic and empirical investigations have shown that the effect of the reflecting droplet formation or a film formation on the baffle wall is essentially dependent upon two dimensionless characteristic numbers, namely, the Reynolds number Re and the Laplace number La. The similarity characteristic quantity Re describes the ratio of the droplet velocity and the droplet diameter to the kinematic viscosity of the fuel. When fuel droplets impinge upon the baffle wall, the velocity component, which is perpendicular to the baffle wall surface, is significant. This velocity component is a function of the angle of impingement. The Reynolds number too is, accordingly, a function of the angle of inclination of the baffle wall. The similarity characteristic quantity La describes the inherent characteristics of the fuel droplet in that its diameter as well as its surface tension and substance density are set in relationship to the square of the fuel viscosity. More detailed investigations have shown that a film formation on the baffle wall occurs when the Reynolds number is:
Rexe2x89xa624xc3x97La0.419. 
To adjust an adequately small Reynolds number for gasoline as fuel, an inclination angle of the baffle wall (that is, of its two component parts) of xe2x89xa670xc2x0 has been shown to be suitable whereby a desired film formation is ensured and an unwanted droplet reflection is avoided.